In integrated circuit (IC) design, a power grid is used to deliver power and ground to transistors as efficiently as possible. The power grid is a power distribution network. In general, a power distribution network should have minimal voltage variation and a high current-carrying capability. For example, if the voltage variation caused by the power grid is increased, the signal strength of the delivered power is decreased, and IR drop is present. Thus, the components (e.g. standard cells or transistors) of the IC cannot work normally, experiencing such problems as function failure, or a reduction of operating speed.
The power grid is formed by using a large amount of metal lines. Electromigration (EM) has long been a problem in power grids used in the semiconductor industry. As electrons pass through a conductor (e.g. a metal wire/line), they tend to drag the metallic ions of the conductor along with them through electrostatic attraction. This results in a slight concentration gradient in the direction of electron flow which in turn sets up an opposing diffusion gradient, so-called back pressure, that tends to move ions towards regions of lower density. If current flows long enough at a sufficiently high current density, the ‘electron wind’ effect dominates and vacancies form which eventually lead to voids and, finally, open circuits, thereby decreasing the reliability of chips.
Therefore, a power grid is important for cell placement in IC design.